1. Field of the Invention
The present invention relates to a three-piece golf ball suitable for both skilled golfers and high handicap players. More specifically, the present invention relates to a three-piece golf ball which has an improved controllability at an approach shot and an improved shot feeling while maintaining excellent flight performance inherent to a solid golf ball.
2. Description of the Prior Art
Hitherto, a thread-wound golf ball and a two-piece golf ball have been generally used. A thread-wound golf ball comprises a core formed by winding a rubber thread around a solid rubber ball (i.e. a solid center) or a rubber bag filled with a liquid (i.e. a liquid center), and a cover enclosing the core. A conventional two-piece golf ball comprises a core made of rubber, and a cover enclosing the core and the core is made of synthetic resin such as ionomer.
A thread-wound golf ball provides a good shot feeling (i.e. soft feel when hitting the ball) because of the cover made of a soft material such as balata and has an excellent controllability due to high spin rate. However, the thread-wound golf ball may not attain a long flight distance in a head wind, especially when hit by golf players with a low club head speed including beginner, female, and the like. Moreover the thread-wound golf ball is poor in durability.
On the other hand, two-piece golf balls have superior durability to thread-wound golf balls and may attain a long flight distance in a head wind, since two-piece golf balls have high resilience and show difficulty to impart intentional spin. Higher resilience can lead to longer flight distance, however the difficulty to impart the spin on the two-piece golf ball causes poor controllability, for example a long run on a green at an approach shot.
A long flight distance is beneficial to high handicap players, and therefore they prefer golf balls having high resilience. Contrarily, excellent controllability is beneficial to more skilled players, especially professional golfers, and therefore they prefer golf balls on which intentional spin can be imparted. Since conventional two-piece golf balls cannot satisfy both the goals of a long flight distance and good controllability, two kinds of two-piece golf balls are produced: one can attain a long flight distance with poor controllability, and the other can provide a good controllability with sacrificing long flight distance.
However, there is a large demand for a golf ball which has a good controllability while maintaining a sufficient flight distance. As a golf ball which meets this demand, three-piece golf balls have been suggested. A three-piece golf ball comprises a core consisting of an center, an outer shell placed on the center, and a cover placed on the outer shell.
For example, Japanese Unexamined Patent Publication No. 59-194760 discloses a three-piece golf ball. In the three-piece golf ball, the hardness of the center is increased from the center point toward a surface of the center, and the specific gravity of the center is larger than that of the outer shell. The three-piece golf ball is liable to spin comparing with a two-piece golf ball, but cannot attain a sufficient flight distance to satisfy golfers who desire a long flight distance, especially when hit by a driver. Japanese Unexamined Patent Publication No. 10-57523 also suggests a three-piece golf ball in which the specific gravity of the center is smaller than that of the outer shell. In the three-piece golf ball, the ratio of deformation amount of the core (the center and the outer shell placing on the center) to that of the center alone (i.e. the deformation amount of the core/the deformation amount of the center alone) is in the range of 0.75 to 1. The three-piece golf ball has a such property that there is small difference between the hardness at a surface of the core and the hardness at the center point of the center, and has high spin rate. As a result, a flight distance is impaired, and therefore golfers who desire a longer flight distance cannot be satisfied with the three-piece golf ball.
In order to solve the problems of conventional three-piece golf balls, the present invention provides a three-piece golf ball which can offer an excellent controllability and a good shot feeling, and also can attain a long flight distance.
The present invention provides a three-piece golf ball which comprises a center having a diameter(d1) of 27 to 37 mm and a specific gravity (a), an outer shell placed on the center and having a larger specific gravity (b) than the specific gravity (a), and a cover placing on the outer shell and having a Shore D hardness of less than 60. The center has a JIS-C hardness (X) at the center point thereof and a JIS-C hardness (Y) at a surface thereof satisfying the equation: (Yxe2x88x92X)xe2x89xa78. The center and the outer shell constitute the core of the three-piece golf ball, and the core has a JIS-C hardness (Z) at a surface thereof being 80 or larger. When a load from 10 kgf as an initial load to 130 kgf as a final load apply to the center and the core respectively, a deformation amount (p) of the center and a deformation amount (q) of the core satisfy the equation: (pxe2x88x92q)xe2x89xa70.5.